This invention relates to engagement pressure control apparatus for torque-transmitting mechanisms and, more particularly, to pressure control apparatus having a modulating pressure override boost scheme.
Automatic shifting power transmissions have a plurality of torque-transmitting mechanisms, which are engaged and/or disengaged to establish various drive ratios through the transmission. During a ratio change, at least one torque-transmitting mechanism is being engaged while the ratio is being changed. The engaging torque-transmitting mechanism undergoes a slipping condition during the ratio change to provide a smooth interchange from one ratio to the other.
The torque-transmitting mechanism generally includes a fluid-operated piston, which is pressurized by the output pressure of an engagement regulator valve. The regulator valve controls the rise in pressure for the clutch piston in a controlled manner, such that a smooth engagement of the torque-transmitting mechanism is achieved. The regulator valve must also provide a high substantially constant pressure after the torque-transmitting mechanism is fully engaged. This is termed the xe2x80x9cholding requirementxe2x80x9d for the torque-transmitting mechanism.
Generally, most transmissions utilize a regulator valve that regulates from substantially zero pressure to the maximum or holding pressure for the torque-transmitting mechanism along a single curve. This means that the regulator valve is sized to produce a maximum pressure that is equal to the holding pressure for the torque-transmitting mechanism. Often this provides for a pressure increase rate, which does not always provide the best shift feel for the transmission. If the regulator valve is sized to provide the best shift feel, then it does not always have sufficient capacity to hold the maximum torque that might be available to the torque-transmitting mechanism.
It is an object of the present invention to provide an improved engagement pressure control apparatus for a torque-transmitting mechanism.
In one aspect of the present invention, the engagement pressure control apparatus provides a modulated pressure rise to perform a shifting event and an increase pressure to provide for maximum holding of the torque-transmitting mechanism.
In another aspect of the present invention, the pressure control apparatus includes a regulator valve, which issues the pressure control function to the torque-transmitting mechanism and a control valve, which issues boost pressure commands to the regulator valve.
In another aspect of the present invention the regulator valve provides a pressure step-down function from a mainline pressure source.
In another aspect of the present invention, the regulator valve has a modulating pressure imposed thereon, which controls the pressure output during the shifting portion of a torque-transmitting mechanism engagement.
In another aspect of the present invention, the control valve is pressurized by either the modulating pressure or the mainline pressure to issue a pressure to the regulator valve which is effective to change the engagement pressure level of the output of the pressure regulator valve to a level required for the maximum torque holding event.